Cutoff walls (COWs) are underground, preferably continuous and typically vertical walls, and may serve both as a foundation (or bearing) wall supporting an overlying structure, such as a pedestrian bridge, and as a diaphragm (or barrier) wall limiting seepage of fluids, such as ground water. Various techniques are known for constructing such underground walls. Very generally, a trench having a generally rectangular section is formed (excavated, dug) in the ground and is filled with cement or a hardening liquid such as cement slurry mixed with excavated earth and sand in the excavated trench to form a soil-cement wall. An exemplary resulting COW wall is also generally rectangular in section, and may have a wide range of dimensions including, but not limited to 100 meters long, 25 meters deep and 1 meter wide (thick).
In a one prior art technique for constructing underground walls, the trench is formed as a number of overlapping, parallel, cylindrical holes which arc drilled vertically into the ground for example, approximately 1 meter diameter holes spaced approximately 0.75 meters center-to-center and extending 25 meters into the ground. Apparatus for drilling and in-situ mixing to construct soil-cement piles (or columns) for soil solidification purposes is well known and shown, for example, in commonly-owned U.S. Pat. No. 5.411,353. Underground walls constructed according to this technique are generally not considered to be “continuous”, and are merely cited herein as contextual prior art.
In a prior art technique for constructing underground continuous walls, an excavator comprises an endless chain cutter (also known as a “trenching body”), resembling a chain saw bar and chain, comprising alternating cutter bits and agitator (mixing) bars arranged on an elongate guide post which extends into the ground. The guide post and endless chain are generally similar, but typically larger, than the endless chain and bar of trenchers for burying pipes, cables and the like, such as from Bobcat™ (e.g., the T136 hydrostatic trencher) and Ditch Witch™ (e.g., 1620, 3500). In this technique, the guide post is penetrated vertically into the ground, typically in a hole which has first been dug or drilled into the ground to the desired depth of the resulting wall, and the apparatus is then advanced (moved) slowly in a line along the surface of the ground. The excavated earth (and sand) is mixed with a hardening liquid such as cement slurry, and is injected into the trench behind the endless chain cutter. Examples of this technique may be found in Japanese Patent Publication No. 5-280043 (Document No. 4-79849) and in Japanese Patent Publication No. 5-280044 (Document No. 4-79850). As described in Document No. 4-79850 (at page 4 thereof):
“[An] underground continuous wall construction method . . . comprises the steps of digging a hole in the ground to a predetermined depth by means of a drilling device, inserting the endless chain cutter of the excavator into the hole, and excavating the ground in a predetermined direction by means of the endless chain cutter and removing the excavated earth and sand, while inserting an injection pipe behind the endless chain cutter to jet a hardening liquid mixed with the removed earth and sand into the excavated hole, thereby filling the excavated hole with any one of soil cement and soil mortar.”
FIGS. 1A, 1B, and 1C illustrate an excavator 100 of the prior art, which is comparable to the excavator described in the aforementioned Document No. 4-79850. The excavator 100 is for forming a trench 102 in the ground 104 for constructing an underground wall. FIG. 1D illustrates an underground (cutoff) wall 160 being constructed in the trench 102.
A starter hole 108 is dug or drilled vertically into the ground 104, typically to the desired resulting depth D to the bottom 106 of the trench 102 being excavated, typically by means of a boring device such as an earth drill (not shown). An endless chain cutter 110 extends vertically into the hole 108. The endless chain cutter 110 comprises an elongate guide post 112 (or “cutter post”), sprockets 114 and 116 at opposite ends of the guide post 112, an endless chain 120 extending around the guide post 112 and the sprockets 114 and 116, and a number of cutter bits 122 and agitator bars 124 alternately arranged on the endless chain 120. It should be understood that any suitable driving means (not shown) may be provided for driving (rotating) the chain 120, such as in the direction indicated by the arrow 126.
The endless chain cutter 110 is elongate, having a top end 110a and a bottom end 110b, and having an overall length S which is greater than the desired resulting depth D of the trench 102 being excavated. For example, an endless chain cutter 110 having an overall length S of 30 meters may be disposed in a hole 108 which is only 20 meters deep. Typically, the underground portion of the endless chain cutter 110 is longer than the portion above the ground, so as to ensure stability and reduce the risk of the endless chain cutter 110 falling down.
The endless chain cutter 110 may be made up of several sections which are assembled together, in a known manner. For example, an endless chain cutter 110 having an overall length S of 30 meters may be made up of six sections—each section having a length of 5 meters. The endless chain cutter 110 is typically assembled in a vertical position, inside the hole 108.
The endless chain cutter 110 is generally rectangular in cross section, having cross-sectional dimensions B and H. The cross-sectional dimension B is in the direction of the length L of the trench 102 being dug (excavated), and the cross-sectional dimension H corresponds to the resulting width W of the trench 102. Typically, the dimension B is greater than the dimension H. For purposes of discussion, the chain 120 itself has a width (not labeled) which may be equal to the cross-sectional dimension H of the chain cutter 110.
Evidently, as best viewed in FIG. 1C, the diameter of the hole 108 which is dug into the ground into which the endless chain cutter 110 is disposed, corresponds to and should be slightly larger than the cross-sectional dimension B of the endless chain cutter 110.
With the endless chain cutter 110 inserted into the hole 108 in the ground 104, it is then moved along the X-axis, as indicated by the arrow 138, to create the trench 102 having a length L. This is accomplished by mounting the endless chain cutter 110 in a suitable manner to a trolley 130 which preferably travels on two elongate rails 132 and 134 which are laid, parallel to one another adjacent and parallel to the trench 102 desired to be dug.
The resulting trench 102, and hence the resulting wall 160 being constructed in the trench 102, is in the general form of a three-dimensional rectangular prism having a depth dimension D which is into the ground in the Z-axis, a length dimension L which is along the ground in the X-axis, and a width dimension W which is also along the ground in the Y-axis. The X-, Y- and Z-axes are mutually orthogonal to one another. As best viewed in FIG. 1C, the length L of the trench 102 is typically much larger than the cross-sectional dimension B of the endless chain cutter 110. For example, the trench 102 and resulting wall 160 may be approximately 100 meters long and, as suggested above, approximately 20 meters deep. By way of further example, the width W of the trench 102 may be approximately 0.5-1.0 meters, the cross-sectional dimension H of the endless chain cutter 110 may similarly be approximately 0.5-1.0 meters, and the cross-sectional dimension B of the endless chain cutter 110 may be approximately 1.0-2.0 meters.
It is generally desirable that the resulting wall is not inclined, but rather is substantially vertical and perpendicular to the surface of the ground—in other words, substantially constant Y-axis coordinates along the length of the trench 102. This is indicated by the right angle symbol 136 in FIG. 1B. To accomplish this, the endless chain cutter 110 is secured to the trolley 130 with a tilt mechanism 140. The tilt mechanism 140 comprises a lug 142 extending from (or secured in a suitable manner to) the elongate guide post 112, a corresponding lug 144 extending from a suitable corresponding position on the trolley 130, a pivot pin 146 pivotably connecting the lug 142 with the lug 144, and a suitable mechanism 148 such as a hydraulic actuator for applying force to the endless chain cutter 110 to control its inclination, about a pivot axis 150, as indicated by the arrows 152 and 154. The pivot pin 146 is parallel to the X-axis to control the Y-axis tilt or inclination of the endless chain cutter 110.
As the trench 102 is being excavated, by advancing the endless chain cutter 110 in the X-axis, a hardening liquid such as cement slurry may be jetted into the excavated space behind the endless chain cutter 110, thereby mixing the hardening liquid with the earth (and sand) in the excavated trench to form a soil-cement wall 160, as shown in FIG. 1D.
FIGS. 2A and 2B illustrate, in side and front views, respectively a typical arrangement of alternating cutter bits 122 and agitator bars 124 of a chain 120. These views are essentially magnified views of corresponding portions of the chain 120 shown in FIGS. 1A and 1B, respectively.
For each cutter bit 122, there are a plurality of individual cutter bits 122a, 122b, 122c, 122d, 122e, 122f, 122g, 122h, 122i. 122j, 122k and 1221, typically identical to one another, an arranged in two rows, each row extending across the cross-sectional dimension H of the endless chain cutter (110) and each row having six cutter bits. Each of the individual cutter bits 122a-122l is fixed to a base plate 222 which is fixed to a link 220 of the chain 120.
For each agitator bar 124, there are a plurality of individual agitator bars 124a, 124b, 124c and 124d, typically identical to one another and suitably arranged in a single row extending across the cross-sectional dimension H of the endless chain cutter (110). Each of the individual agitator bars 124a-124d is fixed to a base plate 224 which is fixed to a link 220 of the chain 120.
The prior art, as described hereinabove, suffers from a number of shortcomings and problems, including the following:                A separate/extra piece of equipment, such as earth drill is needed to start the trench, and the starter hole must be drilled to the full depth of the resulting trench. After drilling the starter hole, the earth drill is superfluous.        It is difficult to assemble the endless chain cutter in a vertical position, which may include assembling it partially within the starter hole.        It is difficult to control the vertical orientation and tilt of the endless chain cutter while moving it to excavate the trench.        It is difficult to maintain good balance for the endless chain cutter throughout the trench-excavating operation.        It is difficult to adjust the endless chain cutter to different trench/wall depths.        The use of specialized equipment, such as the trolley 130 increases the total cost of the overall system. Also, the total weight and power consumption of the overall system is high.        The equipment is not readily adapted to any ground surface-condition, and is difficult to achieve consistency with differing ground conditions (hard soil layer, gravel, cobble, boulder, etc.)        There must be room next to the trench being dug (excavated) for the trolley, which makes it difficult to build a cutoff wall in narrow spaces, such as in the center of a dike having a narrow width.        At the completion of the trenching operation, it is difficult to remove the endless chain cutter.        